The present invention broadly relates to a process for producing a compact billet employing powder metallurgical techniques, and more particularly, a process for producing an elongated densified billet comprised of at least two different alloy compositions metallurgically bonded at their interface including a peripheral outer section and an inner central and axially extending core substantially concentric to each other.
A variety of metal alloys are characterized as having a metallurgical structure in the as-cast condition which renders them extremely difficult to postform to a desired final shape employing conventional forming techniques such as forging or the like. Typical of such metal alloys are the so-called nickel-based superalloys which are generally characterized as having carbide strengthening and gamma prime strengthening in their case and wrought forms containing relatively large quantities of second phase gamma prime and complex carbides in a nickel-chromium gamma matrix. This metallurgical structure contributes to the excellent high temperature physical properties of such alloys but also renders ingots cast from such alloys difficult to postform and rendering them susceptible to macrosegregations resulting in cast billets which are of nonuniform microstructure and possessed of less than optimum physical properties.
Because of the foregoing, powdered metallurgical techniques have now been adopted whereby such alloys are microcast or atomized into a powder of the selected particle size which thereafter is consolidated under high pressre and elevated temperatures into a dense mass approaching 100 percent theoretical density. The resultant densified metallurgical billet is of uniform composition and microstructure.
In the fabrication of rotary components subject to high temperatures under high stress conditions such as gas turbine discs, for example, the desired physical and chemical properties of the outer peripheral portion of the disc defining the blade sections and/or blade attachment section is desirably different from those of the inner or hub section to achieve optimum performance and durability. The blade section of gas turbine discs preferably is comprised of an alloy composition and microstructure which provides for high temperature tensile strength, high temperature creep strength and good corrosion resistance. On the other hand, the central hub section of such turbine discs which are exposed to lower temperatures during service is desirably possessed of high tensile strength, good low cycle fatigue and good crack-growth resistance. The fabrication of a gas turbine disc from a billet which is of substantially uniform composition and microstructure throughout necessitates a compromise between the desired characteristics of the blade section and the hub section to provide a final integral turbine disc possessed of satisfactory performance and durability.
The process of the present invention overcomes the problems and disadvantages as hereinabove set forth by which a composite billet is produced employing powder metallurgical techniques such that selected annular sections thereof are of controlled different alloying composition and/or microstructure thereby optimizing the performance, strength and durability of rotary components fabricated therefrom and providing distinct cost savings and improved performance over similar rotary components comprised of assembled sections of parts composed of different alloy compositions.